1. Field of the Invention
This invention relates to a process and apparatus for sintering of electrodes for fuel cells, in particular, electrodes for molten carbonate fuel cells.
1. Description of Prior Art
The present invention relates to a method and apparatus for preparing porous electrodes for use in a variety of applications. One principal use is for the preparation of porous electrodes to be employed in molten carbonate fuel cells.
Molten carbonate fuel cells typically operate at high temperatures of about 600.degree.-750.degree. C. to convert chemical energy to D.C. electricity. Fuels such as hydrogen, carbon monoxide or methanol react with oxidant gases such as air or oxygen to produce the electrical energy.
Typically, these fuel cells operate in stacks of individual fuel cell units. Each fuel cell unit contains an anode, a cathode and an electrolyte structure separating the two electrodes. The electrode structure is prepared by mixing the component powders thereof with a suitable organic binder and forming the "green" composition into a flattened structure such as a sheet or a tape formed by a tape-casting method. The flattened structure is subsequently sintered at sufficient conditions to bind the particles together into a porous structure. To effect sintering of the "green" electrodes, the electrodes are loaded onto a conveyor means which carries the "green" electrodes into the sintering furnace.
Known sintering methods for making electrodes consist of using either carbon blocks or carbon paper as a support for the "green" electrode as it is conveyed through the sintering furnace. However, carbon blocks have the disadvantage that they are either very fragile or very thick and heavy. In the latter case, they, of course, also have a high thermal mass, thereby affecting the amount of time and energy required for sintering. The use of carbon paper is also problematic due to bubbling and reaction with the electrode and, possibly, with the electrode binder, causing flaws in the sintered electrode. The requirements for a suitable sintering carrier for electrodes to be sintered are that it not impede the shrinkage of the electrode as it is heated, that it not compromise the flatness of the resulting electrode structure and that it not add impurities to the resulting electrode structure.
U.S. Pat. No. 5,110,541 teaches a method for manufacturing a porous electrode for molten carbonate fuel cells in which a mixture of nickel powders and a pulverized aluminum-based intermetallic compound is formed into a slurry, shaped into a sheet or tape and sintered to form the porous electrode. The aluminum-based intermetallic compound, in accordance with the teachings of this patent, provides reinforcement for the porous electrode.
U.S. Pat. No. 4,994,221 teaches a method for producing a carbon electrode in which a carbon material is directly deposited on an electroconductive electrode substrate by chemical vapor deposition and the substrate, coated with the carbon material, is subjected to an electrochemical treatment so that the carbon material is doped with a charge carrier material capable of being reversely intercalated therein and deintercalated therefrom. The electrochemically treated substrate is compressed, forming a thin plate-shaped carbon electrode having a high density.
U.S. Pat. No. 4,460,666 teaches an electrode for an electrolytic cell consisting of an embossed electrically conductive substrate coated with a sintered porous metal powder on at least one major surface thereof. The substrate material is preferably a metallic material such as nickel, stainless steel or nickel-plated steel. The substrate is coated by passing through a container filled with a slurry of metal powder, dried to evaporate water contained in the slurry, and then sintered.
U.S. Pat. No. 5,079,674 teaches an electrode for use in supercapacitors made by adding to an aqueous solution of metal salts porous carbon particles to form a slurry, the metal salts adsorbing onto the porous carbon particles. The metal salts are converted to equivalent metal hydroxides or complex oxides, and the resulting solution is decanted. An emulsion of fluorocarbon polymer is added to the decanted solution and kneaded until the fluorocarbon polymer is fibrillated. The kneaded admixture is formed into a sheet and dried in an oven, the dried sheet then being laminated to one or both sides of a separator.
U.S. Pat. No. 4,202,007 teaches miniaturized integrated circuit devices formed by mixing finely divided particles of a substrate and other chemical additives and casting the resulting mixture into slips to form pliant green sheets. An embryonic conductor pattern is coated on the green sheet which is then sintered.
U.S. Pat. No. 3,679,481 teaches a process for manufacturing sintered carrier-type negative electrodes for alkaline storage batteries.